Reverse roll coating process and apparatus



May 12, 1970 c. R. MURRAY 3,

' REVERSE ROLL COATING PROCESS AND APPARATUS Filed June 5, 1967 2Sheets-Sheet 1 'VENT Ch .MU Y

' AGENT REVERSE ROLL COATING PROCESS AND APPARATUS Filed June 5, 1967 c.R. MURRAY 2 Sheets-Sheet 2 FIG. 5

FIG. 6 PRIOR ART INVENTOR Charles R; MURRAY AGENT United States Patent3,511,696 REVERSE ROLL COATING PRGCESS AND APPARATUS Charles R. Murray,Toronto, Ontario, Canada, assignor, by mesne assignments, to StauiferChemical Company of Canada, Ltd., a corporation of Canada Filed June 5,1967, Ser. No. 643,711

Claims priority, application Canada, June 16, 1966,

963,156 Int. Cl. 1305c N08 US. Cl. 117-111 6 Claims ABSTRACT OF THEDISCLOSURE A reverse roll coating apparatus for applying a coating to atravelling porous or open mesh type web with controlled penetration ofsaid web by said coating. Contact between transfer roll and travellingWeb is prevented by means of a scraper blade or a combination of scraperblade and doctor blade.

This invention relates to web coating and, more particularly, to aprocess and apparatus for coating fabric web by the roll transfer methodwherein there is less penetration of the web.

The use of roll coaters for direct coating of various types of webs iswell known. However, among the numerous roll coaters heretofore used forcoating webs, the most versatile and effective one is known as a reverseroll coater. In essence, this coater comprises two Oppositely rotatinghighly polished metal rolls, one being a metering roll which coatingmaterial is applied to and the other being a transfer roll bearing alayer or coating material of a thickness determined by the size of thegap between the said two rolls. In operation the web to be coated iscontinuously drawn along the surface of the transfer roll to pick up itslayer of coating material by means of a resilient backing roll insuringcontact of the web throughout its full width with the transfer roll.

Reverse roll coaters of the above character suffer from a seriousdisadvantage when used with certain types of webs and coating materialsand this is known as strike through. Indeed with these coaters thecoating material is forced through the web and shows on the reverse sidethereof, such undesirable results being aggravated when working withvarious porous webs such as loosely woven fabrics and with relativelythin and highly penetrative liquid coatings. For this reason, the use ofreverse roll coaters has been restricted, in the past, to the coating ofvery closed weave fabrics or fabrics which had been given a previouscoating of some sort to close up the weave.

It is a general object of this invention to provide a novel process andapparatus for web coating.

A more specific object of the invention is to provide a novel processand apparatus for applying a coating to a travelling porous or open meshtype web with controlled penetration of said web by said coating.

A still more specific object of the invention is to provide a reverseroll coater having a scraper blade or a combination of a scraper bladeand doctor blade to prevent contact between the transfer roll and theweb to be coated and to transfer pre-metered coating material from saidtransfer roll to said web.

These and other objects of the invention will become apparent to thoseskilled in the art upon consideration of the following description andaccompanying drawings. P

In said drawings:

FIG. 1 is a diagrammatic end view of a reverse roll coater incorporatinga scraper blade according to the present invention;

FIG. 2 is a similar view of an embodiment of the invention wherein adoctor blade is combined with the scraper blade;

FIG. 3 is a similar view of a further embodiment of the inventionwherein the doctor blade is a notched doctor blade;

FIG. 4 is a similar view of a still further embodiment of the inventionincorporating a doctor knife;

FIG. 5 is a partly broken away plan view of the notched doctor bladeincorporated in the embodiment of FIG. 3; and

FIG. 6 is a diagrammatic end view of a conventional reverse roll coater.

Referring to the drawings in which like reference numerals designatelike parts, FIG. 6 will be described first, in order to illustrate theconventional reverse roll coating technique.

As shown in FIG. 6, coating material from a pool 5 thereof is metered bymeans of a metering roll 1 and a. transfer roll 2 to a layer 6 ofpredetermined thickness. The transfer roll 2 carrying the coating layer6 to be transferred to the web 4 travelling in the direction of thearrow, rotates in a direction opposite the moving direction of thecontacting surface of the web, thus wiping all of the coating onto theweb in the acute angle between the transfer roll 2 and web 4 which isdriven and maintained in close contact with said transfer roll by meansof a resilient backing roll 3.

As hereinbefore mentioned, this procedure suffers from the seriousdisadvantage of allowing the coating material to strike through the web,especially where the web is made of loosely woven fabric.

In accordance with the present invention penetration of the coatingmaterial through the web can be controlled by arranging a predeterminedclearance gap between the transfer roll and the web and by effectingtransfer of the coating layer from the transfer roll to the web by meansof a scraper blade inserted into said gap in a direction opposite thedirection of travel of the periphery of the transfer roll.

In FIG. 1 which illustrates one embodiment of the apparatus according tothe invention, there are shown all the elements of the conventionalreverse roll coater as hereinbefore described, said elements eacheffecting the same conventional function. Whereas, however, in saidconventional coater, the transfer roll 2 and backing roll 3 are soarranged that at all times, part of the travelling web 4 is brought incontact with the transfer roll, in the coater of FIG. 1 a gap of adefinite size is provided between said transfer roll 2 and web 4 intowhich is inserted a scraper blade 7 in a direction opposite that of themoving coating layer 6 on the transfer roll.

In order to control effectively the penetration of the coating materialinto the web, the gap must be adjusted so as to equal the desiredcoating thickness on the web plus the thickness of the layer on thetransfer roll minus the desired degree of penetration of the coatingmaterial into the web.

The scraper blade which is of standard construction, may be of such athickness as to fill the gap or may be thinner.

In practice, the scraper blade scrapes the premetered amount of coatingmaterial from the transfer roll, thus forming a bank of coating materialwhich is then carried away on the web surface. The initial degree ofpenetration of the coating material into the web is controlled by thepredetermined size of the gap between the transfer roll and the webwhile the final degree of penetration is a 3 function of the viscosityof the coating material which has 'been applied to the web.

In another embodiment of the invention as shown in FIG. 2, the scraperblade 7 is combined with a doctor blade 8. The two blades are arrangedin contiguous relation and so disposed that the working edge of thedoctor blade projects further in the gap than that of the scraper blade.This arrangement is to prevent contact of the scraper blade with the weband to avoid streaks on the coated product caused by web fluff beingtrapped on the scraper blade.

Where it is desired to obtain a lined coating on the web, use is made,instead of the standard doctor blade of FIG. 2, of a notched doctorblade the construction of which is illustrated in FIG. 5. As shown insaid FIG. 5, the working end of the notched doctor blade ischaracterised by having teeth alternating with indentations 11. In thisembodiment of the invention, the structure of which is shown in FIG. 3,the notched blade 9 is arranged in contiguous relation with the scraperblade 7 and so positioned that the bottom of the indentationssubstantially coincides with the working edge of the scraper blade. Theteeth 10 must be long enough to project out of the coating material sothat, in operation, the scraped coating material is forced by virtue ofthe movement of the transfer roll 3 and web 4 through the indentationsbetween the teeth 10 and thereby extruded on the web in the form ofstrips or lines.

FIG. 4 illustrates another modification of the apparatus shown in FIG. 2and shows an embodiment of the invention in which a doctor knife 12 isso positioned in relation with the doctor blade 8 as to provide a slitthrough which the coating material is extruded on the web. As can beseen in FIG. 4, the doctor knife 12 is disposed in such a way that itsworking edge is substantially parallel to and faces the working edge ofthe doctor blade 8. By adjustment of the distance between the twoopposing edges, a gap or slit of definite dimensions is formed throughwhich the coating material from the bank created by the action of thescraper blade 7 is extruded on the web 4 in a layer of a definitethickness.

Although, throughout the drawings, the metering of the coating layersubsequently carried by the transfer roll is effected by means of a roll(metering roll 1), it should be obvious to those skilled in the art thatother means could be used for that purpose such as, for instance, adoctor blade.

The transfer roll 2, backing roll 3, scraper blade 7, doctor blade 8 anddoctor knife 12 are all of standard construction and are made ofsuitable material well known to those versed in the art of coating. Thetransfer roll may conveniently have a polished metal surface while thebacking roll may generally be a rubber roll or a roll the surface ofwhich consists of a resilient natural or synthetic resin. The blades anddoctor knife are generally made of metal.

Although it has previously been specified that the process and apparatusof the invention are especially useful for coating porous or open meshtype web such as loosely woven fabrics, it is to be understood that itis also applicable to coating non-porous webs or strips of all kinds.

It has been found, for instance, that the process and apparatus of theinvention are particularly effective in eliminating pinholing in thepaste film when coating uneven surfaces such as embossed release paper.

The nature of the coating material that can be applied according to thetechnique of the present invention is not critical except that it mustbe such as to enable the material to be formed into a paste or plastisolsuitable for application as a coating. It has been found, for instance,that the invention is of special advantage in the application ofcoatings of vinyl chlorlde resins.

A more complete understanding of the invention will be obtained from thefollowing examples in which the percentages given are by weight unlessotherwise specified.

4 EXAMPLE 1 A plastisol containing 45% of vinyl chloride, 45% ofplasticiser and 10% of filler was prepared in a conventional manner. Thepaste viscosity using a Brookfield viscometer at 2 rpm. was 150,000centipoise. At a shear rate of 9500 secf viscosity was 2500 centipoise.The

coating apparatus used was that shown in FIG. 2 and the web to be coatedwas a fabric.

Cast paste thickness=0.022"

Web paste thickness=0.025"

Metering gap=0.0l6"

Transferzrubber roll ratio=2:1

Transfenrubber roll nip gap=0.065"

Scraper blade thickness=0.050"

Doctor blade thickness=0.0l2"

Distance from edge of scraper blade to doctor blade=%" Under theseconditions a smooth uniform coating was cast onto the fabric at 15y.p.m. with a fabric penetration of approximately 0.005.

A control operation was performed under identical conditions by removingthe scraper and wiping the paste onto the fabric in the conventionalmanner. The paste completely penetrated the fabric wetting thenon-coated side.

EXAMPLE 2 A plastisol containing of vinyl chloride, 40% of plasticiserand 10% of pigment and filler was prepared in a conventional manner.Paste viscosity was 9000 centipoise at a shear rate of 1120 secr Thecoating head was set up as illustrated in FIG. 1 and the web to becoated was embossed release paper.

Cast paste thickness=(l.009"

Web paste thickness=0.007" Metering gap=0.005" Transferzrubber rollspeed ratio=2.8:1 Transfermrbber roll nip gap=0.0l2" Scraper bladethickness=0.012"

The paste was cast on the embossed released paper at a web speed of 14y.p.m. producing a smooth film free from pinholes.

A control operation was performed using the same conditions but removingthe scraper blade from the nip and closing the nip gap 0.002". Thecoating so produced was liberally peppered with pinholes.

EXAMPLE 3 A plastisol containing of vinyl chloride resin, 35% ofplasticiser and 10% of filler and pigment was prepared in a conventionalmanner. Paste viscosity was 100,000 centipoise with a Brookfieldviscometer reading of 2 r.p.m. Paste viscosity at a shear rate of 1120sec? was 12000 centipoise. The machine was set up as shown in FIG. 3 anda notched blade was used having a construction as shown in FIG. 5. Theweb to be coated was a cloth fabric.

Cast paste coating weight-=6% oz./ sq. yd. Web thickness=0.025

Metering gap=0.006"

Transferzrubber roll speed ratio=2:1 Transfer:rubber roll nip gap=0.065"Scraper blade thickness=0.050"

Notched doctor blade thickness=0.0l2

with a uniform layer of said coating material at a predeterminedthickness, a backing roll to support and drive said web to be coated andbring the surface thereof to a predetermined distance from the surfaceof said transfer roll, said distance forming a gap between the surfacesof said web and transfer roll which, when measured on a straight linedrawn between the axes of said transfer and backing rolls, substantiallyequals the thickness of the layer of said coating material on saidtransfer roll plus the desired thickness of the coating on said webminus the desired degree of penetration of said coating material intosaid web, and a scraper blade inserted into said gap, the working edgethereof operating against the direction of the travelling coating layeron said transfer roll.

2. An apparatus as claimed in claim 1 wherein the thickness of thescraper blade is equal to the size of the gap.

3. An apparatus as claimed in claim 1, wherein the scraper blade iscombined with a doctor blade the edge of which penetrates further in thegap than the edge of the scraper blade whereby the edge of the scraperblade is prevented from contacting the surface of the web.

4. An apparatus as claimed in claim 3 wherein the doctor blade has itsworking end formed with alternating teeth and indentation, the bottom ofsaid indentation working in conjunction with the working edge of thescraper blade and the teeth being long enough to project out of the poolof scraped coating material at the nip between the web and the transferroll.

5. An apparatus as claimed in claim 3 including a doctor knife theworking edge of which is substantially parallel to and faces the workingedge of the doctor blade, said edges being adjustable to a predetermineddistance from each other and forming a slit through which the coatingmaterial is extruded on the web from the pool of 6 said coating materialcreated by the action of the scraper blade.

6. A process for continuously coating a web and controlling the degreeof penetration of coating material into said web, which comprisessupplying said coating material in a uniform layer of predeterminedthickness to a continuously driven transfer roll, continuously drawing aweb to be coated in a direction opposite the rotating direction of saidtransfer roll and passing it at a predetermined distance from saidtransfer roll, said distance forming a uniform gap between said web andtransfer roll and being substantially equal to the thickness of thelayer of coating material on said transfer roll plus the desiredthickness of said coating material on said web minus the desired degreeof penetration of said coating material into said web, scraping saidcoating material from said transfer roll at a point within the gap,thereby forming within said gap a bank of coating material from whichsaid coating material is deposited on the web in uniform thickness byvirtue of the continuous movement of the web.

References Cited UNITED STATES PATENTS 1,780,694 11/1930 Alger.2,251,295 8/ 1941 Sheesley. 2,798,820 7/1957 Nelson 11744 X FOREIGNPATENTS 456,377 11/ 1936 Great Britain.

ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, AssistantExaminer US. Cl. X.R.

